The present invention relates to a system for detecting the position of a read-write head on a disk memory such as a magnetic disk and an optical disk.
Recently magnetic or optical disks on which a plurality of concentric circular data tracks are formed have been widely used as one type of memory device. In order to access a selected track on the disk it is desired to accurately and quickly place a read-write head on the selected track. For this purpose, a tracking servo system is required to accurately and quickly position the head at the selected track.
A servo-surface servo (dedicated servo) system is known as one type of conventional servo system which carries servo information on a predetermined side of one of stacked magnetic disks. With a magnetic disk memory apparatus using the servo-surface servo system, there is a disadvantage that an off-track is liable to occur between a servo track and a data track due to the thermal expansion of the disks, and the inclination of a spindle for holding disks and a head carriage. The off-track can be seemingly avoided by lowering the track density of disks. If only one disk is used in a memory apparatus with the servo-surface servo system, only one side thereof is used to store data. Namely, in the case of the servo-surface servo system the utilization of disk surfaces for a high track density is limited.
In order to eliminate the disadvantage of the servo-surface servo system, various types of data-surface servo systems have been developed which utilize servo information recorded on a data surface. As a typical example, an embedded-servo system is proposed wherein a disk data surface is divided into a plurality of data sectors (e.g., 32 sectors), and servo sectors which carry servo information are embedded between data sectors. With the embedded-servo system, disks and servo data processing circuit may be simple in construction. For this reason, the embedded-servo system is used in a cartridge-type magnetic disk storage system.
In the tracking servo system of the magnetic disk apparatus, the present position of a head moving to seek a target track on a disk is detected in accordance with position signals obtained from servo information reproduced from the disk to compute a head speed and the number of tracks up to the target track so that the head is properly controlled in accordance with a head-speed curve which is set in advance. Therefore, it is important to accurately detect the present position of the moving head in accordance with the position signals.
In practice, however, it is difficult to accurately detect the position of the head in accordance with the position signals obtained from the servo information due to the influence of S/N ratio, variation in level signal, and waveform of the position signals.
In the embedded servo system previously described, since the servo-information is obtained only from the servo sectors, continuous positioning control cannot be performed. Therefore, when the head is moved at a high speed, the position signals based on the servo information are obtained incompletely. As a result, the head speed control and the head positioning to the target track will be not performed. Accordingly, the head speed is limited.
With a conventional position detection system, a cylinder pulse is generated each time the position signal crosses the zero level. The cylinder pulses thus generated are counted by a counter to determine the present position of the head. When this position detection system is applied to the embedded-servo system having a two-phase dibit pattern embedded, the cylinder pulse is obtained every two tracks. Therefore, the head speed is limited within one track/sector. This system, therefore, requires a relatively long access time, as compared with the servo-surface servo system.